Polymer compositions which are referred to as "multimodal" are typically multimodal with respect to molecular weight, i.e., the compositions contain two or more molecular weight distributions as may be determined, for example, by the appearance of two or more peaks in a gel permeation chromatogram or the like. However, the term "multimodality" can also refer to other characteristics of a polymer composition as well, e.g., compositional distribution (the distribution of comonomers within a copolymer), tacticity distribution (wherein a polymer composition contains at least two segments of differing tacticity, long-chain branching distribution, and the like. Polymeric compositions that are multimodal are frequently more useful than compositions that are not; for example, multimodal polymer compositions can have improved rheological behavior, higher mechanical strength and increased elasticity relative to corresponding compositions which are not multimodal.
Several processes are known for preparing multimodal polymer compositions. As discussed in U.S. Pat. No. 5,032,562 to Lo et al., one process involves the use of tandem reactors operated in series, so that in a first reactor an olefinic monomer is catalytically polymerized in the presence of hydrogen, with the product then transferred to a second reactor wherein polymerization is conducted in the presence of relatively large amounts of hydrogen. In this way, the higher molecular weight polymer is produced in the first reactor, and the lower molecular weight polymer is produced in the second reactor.
U.S. Pat. No. 5,525,678 to Mink et al. provides a supported catalyst composition for producing a polyolefin resin having a high molecular weight component and a low molecular weight component, wherein the catalyst composition contains a first catalyst which is a metallocene and a second catalyst which is a non-metallocene. The ratio of the high molecular weight and low molecular weight components in the polymeric product is determined by the ratio of the concentration of the two metals in the two-component catalyst composition. In addition, U.S. Pat. No. 4,659,685 to Coleman, III et al. pertains to a two-component catalyst composition for preparing polyolefins having a molecular weight distribution which is multimodal, the catalyst composition comprising a mixture of a supported titanium compound and a separately supported or non-supported organometallic compound.
U.S. Pat. No. 5,032,562 to Lo et al., cited above, also relates to a supported olefin polymerization catalyst composition for producing high density polyethylene ("HDPE") having a multimodal molecular weight distribution. The catalyst composition comprises: (1) a catalyst precursor supported on a porous carrier, and (2) a catalyst activator in the form of a mixture of conventional Ziegler-Natta cocatalysts. Katayama et al., "The Effect of Aluminium Compounds in the Copolymerization of Ethylene/.alpha.-Olefins," in Macromol. Symp. 97:109-118 (1995), provides a similar system for preparing a polymer composition having a bimodal composition using a two-component catalyst comprised of a metallocene (Cp.sub.2 ZrCl.sub.2) and either [Ph.sub.3 C.sup.+ ][B(C.sub.6 F.sub.5).sub.4.sup.- ] or [PhMe.sub.2 NH.sup.+ ][B(C.sub.6 F.sub.5).sub.4.sup.- ].
In addition, certain types of metallocene catalysts have been used to produce polymers having a specific bimodal or multimodal molecular weight distribution.
PCT Publication No. WO92/00333, inventors Canich et al., and EP 416,815 A2, inventors Stevens et al., are also of interest insofar as the references describe metallocene catalysts for preparing polyolefins. Canich et al. describes metallocene catalyst compositions for producing high molecular weight polyolefins having a relatively narrow molecular weight distribution, wherein the catalyst composition is comprised of (1) a metallocene containing a Group IVB transition metal coordinated to a cyclopentadienyl ligand, and (2) a coordination complex such as an anionic complex containing a plurality of boron atoms, which serves as a catalyst activator. The metallocene catalysts described may be mononuclear or binuclear (i.e., containing one or two metal atoms which serve as the active sites); the binuclear compounds dissociate during polymerization. Stevens et al. also pertains to metallocene catalysts to prepare addition polymers, particularly homopolymers and copolymers of olefins, diolefins, "hindered" aliphatic vinyl monomers and vinylidene aromatic monomers. The Stevens et al. catalysts are metal coordination complexes having constrained geometry, and are used in conjunction with a cocatalyst compound to form a complete catalytic system. The constrained geometry of the catalysts is stated to be of key importance insofar as the metal atom in the metallocene presumably is a more "exposed" active site.
Thus, the art provides metallocene catalyst compositions for producing polymers, particular polyolefins, which have a multimodal molecular weight distribution. However, such prior catalysts and catalyst compositions either require two or more components, e.g., two catalysts used in combination, or involve binuclear compounds which break apart into two separate components during the polymerization process (as in the bimetallic catalyst disclosed by Canich et al.), giving rise to potential manufacturing problems, e.g., phase separation or the like, and/or loss of control over the molecular weight distribution of the polymer composition prepared. In addition, the known metallocene catalysts can be relatively difficult and time-consuming to synthesize, requiring expensive equipment, extreme reaction conditions, and multi-step processes which ultimately result in a low yield of the desired product.
Accordingly, there is a need in the art for a simpler way of catalytically preparing multimodal polymer compositions. Preferably, such a process would involve a single catalyst which does not require the presence of a second catalyst, which retains its structure during the polymerization process, and is relatively simple to synthesize. The present invention is directed to such a process, and is based on the use of such catalysts to prepare multimodal polymers, particularly polyolefins. The novel process calls for multinuclear metallocene catalysts having two or more distinct and chemically different active sites. Use of such catalysts allow for a high degree of control over the multimodality of the final polymer composition, and provide for all of the advantages typically associated with metallocene catalysts, i.e., versatility and use in conjunction with a variety of monomer types, the ability to control the degree of vinyl unsaturation in the polymeric product, the capability of providing isotactic or syndiotactic polymers, and the like. The polymerization process may, if desired, be carried out using supported catalysts.